Many electronic components become hot due to the power losses that occur during the electric function of said components. This is the case, for instance, with terminal stage transistors for radio transmitters. The useful life of the components decreases rapidly at elevated temperatures. It is therefore essential to conduct heat away from the components in an effective manner, particularly in the case of the electronic components in equipment where a long useful life is desired, such as telecommunication equipment, for instance. A normal manner of effecting such heat transport is to conduct the heat away from the active element, for instance a transistor chip, through a transfer element which is joined to a device in which heat is transferred to air passing therethrough. Both the efficiency of the thermal conduction from the active element to the heat transfer element and the transfer of heat to the cooling air has direct significance on the temperature difference between the active element and the cooling air.
The following parameters are significant when optimizing air-cooled heat transfer devices: The pronounced or large temperature difference shall prevail between the heat transfer surfaces of said device and the cooling air; the cooling air shall move at a high speed; the heat transfer surfaces shall be large; and the boundary layer shall have a small thickness. By boundary layer is meant here the air layer which lies nearest the heat transfer surface where a gradient of temperature and air velocity prevails.
Those heat transfer devices normally used for cooling electronic components are often of the flanged type which are manufactured by casting, injection moulding, rolling or milling processes. Other known heat transfer devices have the form of corrugated metal sheet. One particular heat transfer device has the form of a cast or moulded device comprising cylindrical pins which extend perpendicularly to a main surface of a pin-carrying plate.
Cylindrical pins which are flushed with cooling air have been found to provide highly effective heat transfer devices, due to a combination of favourable material areas perpendicular to the heat flow in the axial direction of the pins and thin boundary layers alternating with turbulence. One problem is that this design principle often cannot be optimized when practicing the aforesaid manufacturing methods. The combination of pins having a diameter and a pitch which is optimal from a thermal aspect is impossible to manufacture by casting or moulding techniques. In addition to the purely physical requirements, there is often a requirement for low manufacturing costs. Milling constitutes a problem in this regard, because manufacture of the device will then take a relatively long time to complete.
The optimal configuration in this regard, is dependent on the size of the active element and on the pressure and flow rate of the air source used. In one example, where two transistors develop 125 W heat in total, there was used a fan which generated 0.8 m.sup.3 /min. and 200 Pa, wherewith an effective heat transfer was obtained with pins having a diameter of 3 mm, a length of 24 mm, and a pitch or centre-to-centre arrangement of 5 mm. The pins were mounted perpendicularly to one main surface of a plate having a thickness of 4 mm with said surface measuring 83.times.33 mm and thus accommodating 119 cylinders. This configuration cannot be cast, injection moulded, rolled, milled or forged.
The material from which the heat transfer device is made shall have the highest possible thermal conductivity, since this will enable the device to be given smaller dimensions than would otherwise be possible, which in turn results in shorter conduction paths and smaller temperature differences. The cast aluminium alloys usually used have about 150 W/m.degree. C., pure aluminium about 200 W/m.degree. C., and copper and silver about 400 W/m.degree. C. Pure copper can therefore be used advantageously for the manufacture of compact heat transfer devices, but is still more discriminating in the choice of manufacturing methods.
When soldering cylindrical copper pins to a copper plate, the distribution of the solder, among other things, creates a problem. Due to the high thermal conductivity of copper, a pin cannot be soldered to the copper plate without melting adjacent pins. This means that the solder joint alone cannot be used as a fixation means when soldering one pin at a time. If several pins are to be soldered in position simultaneously and these pins are held in position on the copper plate by means of a jig or like fixture, the solder will first melt and wet a given part of said surface. This area of the plate and the pins first wetted by the solder will then suck solder from the surroundings and the solder will rise up between the relatively densely packed pins as a result of capillary action. This means that the solder layers will have greatly varying thicknesses, which presents a problem.